1. Field of the Invention
The present invention relates to a light emitting display, and more particularly to an organic light emitting diode (OLED) display utilizing an organic material to emit light.
2. Discussion of the Related Art
Generally, an OLED display emits light by electrically exciting an organic compound. Such OLED displays include N×M organic light emitting pixels arranged in the form of a matrix, and display images by driving the organic light emitting pixels using voltage or current. As shown in FIG. 9, each organic light emitting pixel has a structure which includes an anode electrode layer (e.g., indium tin oxide (ITO)), an organic thin film, and a cathode electrode layer. The organic thin film has a multi-layer structure including an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL), and achieves an improved balance between electrons and holes, and thus, an enhancement in light emitting efficiency. The organic thin film also includes an electron injecting layer (EIL) and a hole injecting layer (HIL).
The OLED display panel may be driven using a passive matrix type driving method or an active matrix type driving method using thin film transistors (TFTs). In accordance with the passive matrix type driving method, anodes and cathodes orthogonal to each other are arranged so that desired lines may be selected and driven. In accordance with the active matrix type driving method, thin film transistors are coupled to respective ITO pixel electrodes in an OLED display panel so that the OLED display panel may be driven by a voltage maintained by the capacitance of a capacitor coupled to the gate of each thin film transistor.
The conventional OLED display includes a plurality of sub-pixels having distinct colors so that a spectrum of colors may be expressed by combining colors emitted from the plurality of sub-pixels. Conventionally, pixels are provided having sub-pixels for red, green, and blue; thus a spectrum of colors may be expressed by the pixels by using a combination of the red, green, and blue sub-pixels.
FIG. 1 shows a circuit diagram for representing one of N×M pixels as a conventional pixel circuit, equivalently representing a pixel arranged in a first row and a first column.
As shown in FIG. 1, a pixel 10 includes three sub-pixels 10r, 10g, and 10b. The sub-pixels 10r, 10g, and 10b respectively include OLED elements OLEDr, OLEDg, and OLEDb for respectively emitting red, green, and blue lights. Where sub-pixels are arranged in a stripe pattern, the sub-pixels 10r, 10g, and 10b are respectively coupled to data lines D1r, D1g, and D1b, and commonly coupled to a scan line S1.
The sub-pixel 10r for emitting a red light includes two transistors M1r and M2r, and a capacitor C1r for driving the OLED element OLEDr. The sub-pixel 10g for emitting a green light also includes two transistors M1g and M2g, and a capacitor C1g. The sub-pixel 10b for emitting a blue light also includes two transistors M1b and M2b, and a capacitor C1b. Operations of the sub-pixels 10r, 10g, and 10b correspond to each other; accordingly, only the operation of the sub-pixel 10r will be described in detail below.
The driving transistor M1r is coupled between a power voltage VDD and an anode of the OLED element OLEDr, and transmits a current for emitting light to the OLED element OLEDr. The cathode of the OLED element OLEDr is coupled to a voltage Vss which is less than the power voltage VDD. The driving transistor M1r may be controlled by a data voltage applied through a switching transistor M2r. At this time, the capacitor C1r is coupled between a source and a gate of the transistor M1r, and maintains an applied voltage for a predetermined period. A gate of the transistor M2r is coupled to the scan line S1 for transmitting a on/off selection signal, and a source of the transistor M2r is coupled to the data line D1r for transmitting a data voltage corresponding to the sub-pixel 10r for emitting a red light.
A data voltage VDATA from the data line D1r is applied to the gate of the transistor M1r when the switching transistor M2r is turned on in response to a selection signal applied to the gate of the transistor M2r. A current IOLED flows to the transistor M1r which corresponds to a voltage VGS charged between the gate and the source by the capacitor C1r, and the OLED element OLEDr emits light corresponding to the magnitude of the current IOLED. At this time, the current of IOLED flowing through the OLED element OLEDr is given as Equation 1.
                              I          OLED                =                                            β              2                        ⁢                                          (                                                      V                    GS                                    -                                      V                    TH                                                  )                            2                                =                                    β              2                        ⁢                                          (                                                      V                    DD                                    -                                      V                    DATA                                    -                                                                                V                      TH                                                                                          )                            2                                                          [                  Equation          ⁢                                          ⁢          1                ]            
where VTH denotes a threshold voltage of the transistor M1r, and β denotes a constant.
In the pixel circuit shown in FIG. 1, a current corresponding to the data voltage is supplied to the OLED element OLEDr, and the OLED element OLEDr emits light with a brightness corresponding to the supplied current. At this time, the applied data voltage may have various values within a predetermined range in order to express predetermined gray scales.
As shown, the OLED display includes a pixel 10 including three sub-pixels 10r, 10g, and 10b. The respective sub-pixels include a driving transistor, a switching transistor, and a capacitor for driving an OLED element. A data line for transmitting a data signal and a power line for transmitting a power voltage VDD are formed for each sub-pixel. Accordingly, the OLED display must include a great number of lines and circuits for driving the pixels. These lines are difficult to arrange in a limited display area, and the aperture efficiency corresponding to an emitting pixel area is reduced. Therefore, it is desirable to develop a pixel circuit for reducing the number of lines and elements for driving a pixel.